This application claims the priority benefit of Taiwan application ser. no. 89121547, filed Oct. 16, 2000.
1. Field of Invention
The present invention relates to a laser fuse for memory. More particularly, the present invention relates to a type of laser fuse capable of cutting off any input voltage to a memory unit even when the laser fuse is only partially severed.
2. Description of Related Art
In general, a semiconductor memory unit contains a plurality of fuses serving as a pre-starting circuit to be replaced with some defect circuits. Hence, whether or not the safety fuse can be completely broken (or burnt) is a critical issue in the design of memory unit.
FIG. 1 is a schematic cross-sectional diagram showing the structure of a conventional safety fuse. As shown in FIG. 1, a safety fuse 12 is embedded within a p-type substrate 10. There is a laser window 14 above a portion of the fuse 12. The safety fuse 12 is melted away by aiming a laser beam into the laser window 14. Ultimately, a gap is formed in the central portion 16 of the fuse 12 rendering the two ends Cxe2x80x2 and Dxe2x80x2 of the safety fuse 12 open.
However, the laser beam may be slightly offset or beam intensity may be too low due to some design errors. FIG. 2 is a top view showing an incompletely cut safety fuse of a conventional safety fuse structure. As shown in FIGS. 2 and 1, a laser beam aims at a section of the laser window 14. A clean cut of the safety fuse 16 is not obtained because of the laser beam is too weak or the beam is offset. Due to the offset of laser beam, a portion of the safety fuse 20 remains outside the cutting beam 18. Consequently, the safety fuse 16 is still partially linked after a laser-cutting operation.
FIGS. 3A and 3B shows another type of safety fuse structure and the way in which the fuse is incompletely broken after a laser cutting operation. As shown in FIG. 3A, a polysilicon layer 24 that serves as an etching barrier layer is normally formed underneath a laser window 22 but above a safety fuse 26. As shown in FIG. 3B, a laser beam 27 acts on the laser window 22. Although the laser beam 27 is able to cut off the polysilicon layer 24 and the safety fuse 26 completely, residual material from the polysilicon layer 24 will flow into the safety fuse 26 area. Due to the presence of residual polysilicon material, the two ends of the safety fuse 26 may reconnect electrically.
Accordingly, one object of the present invention is to provide a laser fuse inside a memory unit capable of preventing the formation of a partially cut fuse. Hence, an input terminal and an output terminal of the memory unit are in an open state condition after a laser-cutting operation even when the laser beam is slightly offset or power level is slight too low.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a laser fuse between a memory unit and a power source. The safety fuse includes a conventional safety fuse and an NMOS transistor.
The conventional safety fuse has a first voltage source and a second voltage source, which respectively have a low voltage and a high voltage. The second voltage source has a laser window for cutting the safety fuse with a laser beam.
The NMOS transistor has an operating threshold voltage. The gate terminal of the NMOS transistor is connected to a point between the two input terminals of the safety fuse. One source/drain terminal of the NMOS transistor is connected to the input terminal while the other source/drain terminal of the NMOS transistor is connected to the output terminal of the memory unit.
In operation, the low voltage is controlled to a level below the threshold voltage. Hence, the NMOS transistor is incapable of operating and the source/drain terminals are in an open state after the safety fuse is cut by a laser beam. If the laser beam only cuts away a portion of the safety fuse, resistance at the second voltage source is raised. The gate terminal of the NMOS transistor receives a voltage lower than the threshold voltage. Therefore, the NMOS transistor is still incapable of operating and the source/drain terminals are in an open state. In other words, even if the safety fuse is only partially cut, an open state condition between the memory unit and the power source is still ensured.
The safety fuse can be made from a metallic material such as copper or aluminum or a non-metallic material such as polysilicon. The gate terminal of the NMOS transistor is connected to a point between the two terminals of a safety fuse using a metallic junction. The first voltage source of the safety fuse is connected to a low voltage such as a zero volt to ensure that the NMOS transistor remains in a cut-off state once the safety fuse is broken.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.